I have been with the same guy for eight years, and we have been engaged for four years. We just recently bought a house together.
On my way back home from a business trip, he texted me and said he was unhappy and wanted to end everything. He doesn’t want to try to work on things. He says he loves me, but, can’t do it anymore. We decided to sell the house, and one week later, he has started to date a girl he used to sext all the time, apparently completely unrelated (according to him).
I love him, and want to be with him, but, there is nothing I can do if he doesn’t want to work on things. He never gave me a concrete reason besides he is just unhappy. If he really loved me, I’d like to think he never would have done it like that, but, I am having issues dealing with it.
He says he wants to be friends, and that we could possibly get back together later on, but, is someone like that worth it? Advice, please?
LemonVibe 
Definitely not worth it. For your own well being, I would advise you cut off all contact, and take care of you for a bit! Start a new hobby or do some volunteering- I promise you will be ok! Do some fun stuff.
He wants to be with her and keep you around in case it doesn’t work out with her. He’s not “unhappy” he’s being selfish. Trust, I’ve been there. You’ve become too “safe” and he thinks he can just do what he wants, you won’t leave – so that’s exactly what you should do. You deserve better than this. Drop him immediately and don’t look back.
I totally agree with the other comments about this: he’s left you because he wanted to date her, he wants to keep you around as a backup. Don’t let him do it, move on, cut all contact and you will be alright! You can do it!